Interpretive Summary: Bighorn sheep have had lethal pneumonia when housed together with domestic sheep, but an important question has been which pathogens might play important roles in the lethal bighorn pneumonia. One potentially important bacterium is Mycobacterium ovipneumoniae, and we tested its importance by housing bighorn sheep together with domestic sheep known to be free of M. ovipneumoniae. Instead of rapid deaths of bighorn sheep observed in previous co-housing studies, only one bighorn sheep died, and it only did so 3 months after the co-housing experiment began. Three other bighorn sheep remained healthy after more than 100 days of co-housing. These results were very different than previous studies, and they suggested that M. ovipneumoniae plays a key role as a pathogen in bighorn sheep pneumonia. Now that the importance of M. ovipneumoniae has been confirmed experimentally, future research is warranted to identify ways to mitigate transmission and disease due to this pathogen.

Technical Abstract:
To test the hypothesis that Mycoplasma ovipneumoniae is an important agent of the bighorn sheep (Ovis canadensis) pneumonia that has previously inevitably followed experimental commingling with domestic sheep (Ovis aries), we commingled M. ovipneumoniae–free domestic and bighorn sheep (n=4 each). One bighorn sheep died with acute pneumonia 90 days after commingling, but the other three remained healthy for >100 days. This unprecedented survival rate is significantly different (P=0.002) from that of previous bighorn-domestic sheep contact studies but similar to (P>0.05) bighorn sheep survival following commingling with other ungulates. The absence of epizootic respiratory disease in this experiment supports the hypothesized role of M. ovipneumoniae as a key pathogen of epizootic pneumonia in bighorn sheep commingled with domestic sheep